An old technique of passive dosimetry with deferred reading utilizes a sensitive charge comprising silver-based films (that is to say photographic emulsions) onto which are sometimes applied absorbent sheets of paper, plastics material or metal; this sometimes referred to as “dosifilm”. Their implementation is demanding and complex, while being single-use, whereas their response depends on numerous parameters. These films have a detection threshold of at least 0.3 mGy and can measure doses up to 20 Gy. These films do not therefore meet the current needs relating to individual dosimetry, since (in France) the order of Dec. 31, 2004 has set the detection threshold at 0.1 mSv (or 0.1 mGy of X and gamma radiation).
Another technique, which appeared in the 1950's, employs a dosimeter with a thermoluminescent sensitive charge, i.e. a thermoluminescent dosimeter (abbreviated to TLD) using lithium fluoride doped with magnesium (denoted LiF:Mg, and more commonly called “FLi”), alumina, calcium fluoride activated with manganese (F2Ca(Mn), in particular commercialized under the name TLD400), lithium boride (in particular commercialized under the name TLD800), natural fluorite, etc. Reading the TLD is possible only once, but it is re-usable; it is able to measure cumulative doses comprised between 10 μGy and several Gy. U.S. Pat. No. 5,083,031 describes a personal thermoluminescent dosimeter using the principle of absorbent filters to discriminate the nature of the incident ionizing radiation. U.S. Pat. No. 3,582,653 describes an X-ray spectrometer based on alternating capsules of FLi and filter screens which measure quantities proportional to the dose absorbed behind the screens and deduces the spectrum of the incident radiation therefrom by mathematical manipulation.
Another technique employs dosimeters with a radio-photoluminescent sensitive charge, in particular a radio-photoluminescent dosimeter (abbreviated to RPL) which are differentiated from TLDs by the fact that they use certain categories of doped glass, and that the radiative recombination of the electrons trapped in the defect centers, called color centers, is induced by photo-stimulation in the UV range, in practice by laser (as of the 1980's). Dosimeters of this type, provided by the IRSN (standing for Institut de Radioprotection and de Sûreté Nucléaire in French, or Nuclear Safety and Radioprotection Institute in English) use glass doped with a silver-based compound, and comprising three superposed layers of glass and filters (of plastic and metal), which provides 15 measurement ranges in analytical reading (5 in routine use) and to provide an indication as to the nature of the energy of the ionizing reduction concerned (from 10 keV to 10 MeV, for photons). Reading is non-destructive, which enables several readings of the same sensitive charge irradiated by a given dose, or intermediate measurements over long periods of exposure to the radiation at issue. In practice, the minimum sensitivity threshold is 20 μGy and the dose measured may be up to 10 Gy.
Another technique employs dosimeter with optically stimulated luminescence (or OSL in abbreviation), which appeared at the end of the 1990's, of which the active component of the sensitive charge belongs to the family of radiophotoluminescent materials; in practice this is carbon doped alumina (Al2O3:C). Under radiation, the electrons are trapped in the crystal defect centers formed by the carbon atoms. At ambient temperature, the charges may remain trapped for several days; deferred reading of the cumulative energy is carried out by photostimulation by means of a flash emitting green light, the intensity of the characteristic blue light peak being proportional to the cumulative energy. This dosimeter may be re-used after resetting, by heating or by optical illumination. Its measurement range is comprised between 10 μGy and 10 Gy. U.S. Pat. No. 7,420,187 describes an example embodiment thereof as an individual dosimeter.
Another, more recent technique, is based on radiophotoluminescent materials, in particular barium fluoro-halides doped with europium, of (BaFBr:Eu2+) type, used in producing flexible radiographic screens, commonly referred to as “photostimulable phosphor plates” or PSP plates; these screens are also called “photostimulable storage phosphor plates”. As such, PSP plates are commonly used, in particular for radiography in the medical field. They are sometimes associated with intensifying screens (see the paper “Evaluation of a computed radiography system for megavolt photon beam dosimetry” by Olch et al, Med Phys 32(9), September 2005, 2987-2999). The signal-to-noise ratio and the detection threshold of a PSP plate imager may be improved by using a stack of several cells individually constituted by “metal sheet/PSP plate” pairs (see, in particular, “Improvement of signal-to-noise and contrast-to-noise ratios in dual-screen computed radiography”, by Shaw et al, Med. Phys. 24, 1997, 1293-1302). A cassette adapted to receive such a stack of pairs is described in document WO 2009/030833. The addition of the images, arising from redundancy of the radiographic recording on the different cells, enables the signal-to-noise ratio to be increased in the resulting image. The signal level obtained is roughly proportional to the number n of screens used whereas the noise level is proportional to a square root of that number of screens (cf. “La mesure en détonique; R&D en radiographie éclair AIRIX”, by Abraham et al, Chocs journal No. 38 by the CEA-DAM pp 18-28 (2010). However, the application of this technique is complex to implement since PSP plates must be carefully positioned relative to each other, possibly with interleaved screens; they must then be scanned, one after the other; lastly, the images must be spatially re-adjusted relative to each other without error. All these manipulations are long, difficult to automate and lead to high risks of error; this technique is therefore currently used only very occasionally by research laboratories and has not given rise to industrial applications.
Although PSP plates, RPLs and OSLs have in common the implementation of radiophotoluminescent materials, they are differentiated by the material of PSP plates being flexible and able to be manufactured in the form of an ultra-thin layer (typically of the order of 100 μm) which can be deposited or bonded onto a support of some kind, in particular plastic or paper or the like.
None of the existing dosimeters (TLD, OSL, RPL or PSP plate) meets all current needs.
Thus, for photon beams of energy higher than the MeV, reliable quantification of the dose requires them to be placed in electron equilibrium, that is to say in a phantom material of density and thickness perfectly adapted to the energy spectrum of the radiation; the latter must be known, which is not generally the case. Furthermore, their sensitivity threshold is still too high to for the rapid measurement of the dose corresponding to natural environments. Several days of exposure are often required to obtain satisfactory measurements, which can be a very considerable drawback in a crisis situation where radiological contamination is suspected for example; furthermore, this requires multiple operations of intervention in the field to set up the dosimeters to return to read them. The dose limit for the “public” is typically 0.5 μSv/h (80 μSv/month on the basis of 2000 h/year of average operation of the installation. It is thus necessary to operate the installation for several hours to exceed the detection thresholds of a dosimeter in the context of a check, and thus to know whether a normal dose has been exceeded.
Furthermore, obtaining a precise measurement of the quality of the radiation (energy of the particles and dose in Kerma) is difficult. This requires setting up a set of dosimeters juxtaposed against each other or placed one behind the other with different thicknesses of interleaved materials to filter the radiation and attain electron equilibrium. Locating the position of each dosimeter and analyzing the results is painstaking and liable to error when manipulations are carried out (see in particular the paper “Reconstruction of high-energy bremsstrahlung spectra by numerical analysis of depth-dose data” by Otto Sauer et al, Radiotherapy and Oncology, Vol 18, Issue 1, May 1990, pp 39-47, or the paper “Reconstruction of 6 MV photon spectra from measured transmission including maximum energy estimation” by Colin R Baker et al, Phys Med Biol 42, pp 2041-2051 (1997) or the paper “Robust megavoltage X-ray spectra estimation from transmission measurements” by Marian Manciu et al, Journal of X-ray Science and Technology, 17 pp 85-99 (2009)).
As regards PSP plate readers, sophisticated image digitization systems (2D reading of the screens by laser) are commercially available. These systems are all composed of finely adjusted optical and optoelectronic elements enabling the digitization of images of relatively large size relative to those of a PSP plate composing the dosimeter. These systems are thus difficult to transport to be adapted to the field and are oversized and costly if the user merely uses the dosimeters.